This application is based on Japanese Patent Application No. 2001-233140 filed on Aug. 1, 2001, the contents of which are incorporated hereinto by reference.
1. Field of the Invention
The present invention relates in general to a grinding wheel having an outer annular grinding portion and an inner core portion located radially inwardly of the grinding portion and functioning primarily to support the grinding portion, and a process of manufacturing the grinding wheel and a process of reclaiming the grinding wheel.
2. Discussion of Related Art
Annular grinding wheels are widely used in various fields of industry, for performing a grinding operation by rotation of the grinding wheel. Examples of such annular grinding wheels include a vitrified-bond wheel wherein abrasive grains are held together with an inorganic bonding material, and a resinoid-bond wheel wherein the abrasive grains are held together with a synthetic resin. In the annular grinding wheel manufactured in the prior art, an inner portion of the grinding wheel is generally formed, as a core portion integrally with the outer grinding portion, of the same material as the annular grinding portion. When the service life of the grinding wheel has been reached with a considerable amount of wear of the outer grinding portion, the entirety of the grinding wheel is discarded, so that the material of the inner core portion which has not been used for the grinding operations is wasted, without reusing or reclaiming the inner core portion.
On the other hand, there has been developed a grinding wheel consisting of an outer annular grinding portion, and an inner core portion which is located radially inwardly of the outer annular grinding portion and functioning primarily to support the grinding portion. Examples of this type of grinding wheel include a grinding wheel having a central core portion formed of a steel, aluminum or other metallic material as disclosed in JP-A-50-47289, and a grinding wheel having a central core portion formed of a thermosetting resin. These grinding wheels provide an improvement in the strength of the core portion. The present inventor made a research on a possibility of reclaiming the grinding wheel, by separating the outer annular grinding portion and the inner core portion from each other, after the service life of the grinding wheel has been reached with a considerable amount of wear of the outer grinding portion. This research was made in an effort to find a practical way of reusing the core portion of the grinding wheel, in view of a recent increasing need for recycling the materials in various fields of technology, to reduce the amount of discarding of the materials and avoiding an abuse of the natural resources.
However, it is relatively costly to reuse the metallic core portion, since the reuse requires treatments or finishing operations to eliminate pits or other damages on the surfaces of the metallic core portion, which have been generated during a long use of the grinding wheel. On the other hand, the reuse of the core portion formed of a thermosetting resin is conventionally considered substantially impossible since it is difficult to recover the original properties of the thermosetting resin once it is deteriorated. Thus, practical methods for reusing or reclaiming the grinding wheel are not available in the prior art.
It is therefore a first object of the present invention to provide a grinding wheel which has an outer annular grinding portion and an inner core portion located radially inwardly of the annular grinding portion and functioning primarily to support the annular grinding portion, and which can be effectively reclaimed. A second object of the invention to provide a process of manufacturing such a grinding wheel. A third object of the invention is to provide a process of reclaiming the grinding wheel of the type indicated above.
In the prior art, the core portion of the grinding wheel is formed of a metallic material or a thermosetting resin, as described above. The metallic material and thermosetting resin are conventionally selected for the core portion, for their high tensile strength high enough to permit the grinding wheel to be operated under a comparatively severe operating condition, for example, to be rotated at a peripheral speed as high as about 60-80 m/s, for performing a high-speed grinding operation. In selecting the material for the core portion, the present inventor paid attention to the specific tensile strength of the material, which is equal to the tensile strength (MPa) divided by the specific gravity. The inventor found that the use of a material having a specific tensile strength of at least 12 MPa for the core portion enables the core portion to exhibit a strength high enough to withstand a load during a severe grinding operation of the grinding wheel. The inventor further found that the use of a thermoplastic resin having a specific tensile strength for the core portion makes it possible to reclaim or recycle the core portion of the used grinding wheel for manufacturing the core portion of a new grinding wheel, while enabling the core portion to exhibit a sufficiently high strength. The present invention was made in the light of the inventor""s findings described above.
The first object indicated above may be achieved according to the first aspect of the principle of this invention, which provides a grinding wheel comprising an outer annular grinding portion, and an inner core portion located radially inwardly of the annular grinding portion to support the annular grinding portion, the core portion being formed of a composition including as a major component a thermoplastic resin which has a specific tensile strength of at least 12 MPa.
The grinding wheel constructed as described above according to the present invention has a sufficiently high strength at its inner core portion. In addition, the core portion of the grinding wheel whose service life has been reached with a considerable amount of wear of the grinding portion can be reused as a material for the core portion of the new grinding wheel. Namely, the used grinding wheel is broken down into the outer grinding portion and the inner core portion, and the core portion is crushed into pellets, which are then heated into a molten state in which the thermoplastic resin can be reclaimed as the material for the core portion of the new grinding wheel. Further, the outer grinding portion of the used grinding wheel can also be reclaimed as a material for the grinding portion of the new grinding wheel, by crushing the grinding portion of the used grinding wheel. The amounts of fresh materials required for forming the grinding and core portions of a new grinding wheel can be significantly reduced by reclaiming the materials of the grinding and core portions of the used grinding wheel, so that the cost of manufacture of the new grinding wheel can be effectively reduced, while the used grinding wheel as a whole can be completely recycled.
Further, the use of the thermoplastic resin for the core portion is effective to improve the grinding ratio of the grinding wheel, smoothness of the workpiece surface ground by the grinding wheel, and economy of electric power consumption by a grinding machine, owing to the higher degree of impact resistance or vibration absorption capacity of the thermoplastic resin than the conventionally used materials such as abrasives, metals, and thermosetting resins. In this respect, the grinding wheel having the core portion formed of the thermoplastic resin according to the present invention is advantageous over the prior art grinding wheel using the above-indicated materials for the core portion.
In one preferred form of the grinding wheel according to the first aspect of this invention, the composition of the core portion includes a fresh thermoplastic resin, and a mass of pellets which is obtained by crushing the core portion of a grinding wheel of the invention a service life of which has been reached. Preferably, the core portion is formed by injecting the composition in a molten state into a mold cavity formed to form the core portion. In the above form of the invention, the core portion of the grinding wheel whose service life has been reached with a considerable amount of wear of the grinding portion is reclaimed as a material for the core portion of the new grinding wheel, so that the grinding wheel can be manufactured at a reduced cost, with reduced amounts of fresh materials required for the grinding wheel, while enabling the core portion to have a sufficiently high strength.
In another preferred form of the grinding wheel of the invention, the thermoplastic resin is selected from a group consisting of engineering plastics including polyamide, polyester and polycarbonate. These engineering plastics which have been used as a material for structural members have a specific tensile strength of at least 12 MPa, and high degrees of resistance to water and oil. In this respect, polyamide, polyester, polycarbonate and other engineering plastics can be suitably used as a major component of the inner core portion of the present grinding wheel.
In a further preferred form of the grinding wheel of the invention, the composition of the core portion further includes, as an aggregate, an inorganic filler such as a glass fiber and an abrasive material. Where the grinding wheel is operated under a relatively severe condition, the core portion of the grinding wheel is required to exhibit relatively high degree of strength and elastic modulus. In this case, the composition of the core portion preferably includes an inorganic filler such as an abrasive material or a glass fiber, in addition to the thermoplastic resin provided as a major component of the composition. The use of the inorganic filler is effective to increase the strength and elastic modulus of the core portion.
The second object indicted above may be achieved according to a second aspect of this invention, which provides a process of manufacturing a grinding wheel having an outer grinding portion and an inner core portion which is located radially inwardly of the annular grinding portion to support the annular grinding portion, the process comprising the steps of;
crushing the core portion of the grinding wheel into pellets to be reclaimed after a service life of the grinding portion has been reached;
preparing a composition for forming the core portion, the composition including a mass of the above-indicated pellets and a fresh thermoplastic resin having a specific tensile strength of at least 12 MPa; and
forming the core portion by using the composition in a molten state.
The process according to the second aspect of this invention not only permits a manufacture of the grinding wheel having the inner core portion which has sufficiently high degrees of mechanical strength and durability, but also makes it possible to reclaim the inner core portion of the used grinding wheel, by separating the grinding and core portions of the used grinding wheel from each other and crushing the core portion into pellets and heating the pellets into a molten mass. The used grinding wheel is a grinding wheel a service life of which has been reached with a considerable amount of wear of the grinding portion. According to the present process, the required amounts of the fresh material for forming the core portion of the new grinding wheel is effectively reduced, so that the cost of manufacture of the grinding wheel is accordingly reduced, owing to the recycling of the used grinding wheel.
The third object indicated above may be achieved according to a third aspect of this invention, which provides a process of reclaiming materials of a grinding wheel having an outer annular grinding portion, and an inner core portion located radially inwardly of the annular grinding portion to support the annular grinding portion, the process comprising the steps of:
separating the outer annular grinding portion and the inner core portion of the grinding wheel from each other;
crushing the annular grinding portion into grains of a predetermined size, for reclaiming a mass of the grains as a part of a composition for forming the annular grinding portion of a new grinding wheel; and
crushing the core portion into pellets, for reclaiming a mass of the pellets as a part of a composition for forming the core portion of the new grinding wheel.
In the process according to the third aspect of this invention, the grinding wheel which has been used for its expected service life is not discarded, but is broken down into the outer grinding and inner core portions, so that the core portion is crushed into a mass of pellets to be reclaimed as a part of the composition for the core portion of a new grinding wheel, while the grinding portion is crushed into a mass of grains of a predetermined size to be reclaimed as a part of the composition for the grinding portion of the new grinding wheel. Accordingly, the amounts of fresh materials required for manufacturing the new grinding wheel are reduced, so that the grinding wheel can be manufactured at a reduced cost, with complete recycling of the used grinding wheel. The present process is significant in the environmental protection.